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Abstract:

A method and apparatus involve: routing first radiation and second
radiation respectively having first and second wavelengths that are
different along respective first and second optical paths; reflecting the
first radiation with an optical component as the first radiation is
traveling along the first optical path; and reflecting the second
radiation with the optical component as the second radiation is traveling
along the second optical path, the optical component causing a first
optical path length traveled by the first radiation along the first
optical path from arrival at to departure from the optical component to
be shorter than a second optical path length traveled by the second
radiation along the second optical path from arrival at to departure from
the optical component.

Claims:

1. An apparatus comprising a path-length adjusting optical component that
is reflective to each of first and second wavelengths, said first and
second wavelengths being different and traveling along respective first
and second optical paths that each include a reflection by said optical
component, wherein a first optical path length traveled by said first
wavelength along said first optical path from arrival at to departure
from said optical component is shorter than a second optical path length
traveled by said second wavelength along said second optical path from
arrival at to departure from said optical component.

2. An apparatus according to claim 1, wherein said optical component is
reflective to a third wavelength that is different from each of said
first and second wavelengths, said third wavelength traveling along a
third optical path that includes a reflection by said optical component,
a third optical path length traveled by said third wavelength along said
third optical path from arrival at to departure from said optical
component being longer than said second optical path length.

3. An apparatus according to claim 1, wherein said optical component
includes: a first reflective section that reflects said first wavelength
and transmits said second wavelength; and a second reflective section
that reflects said second wavelength, said second optical path extending
through said first reflective section toward said second reflective
section, and then through said first reflective section away from said
second reflective section.

4. An apparatus according to claim 3, wherein an optical distance to said
second reflective section along said second optical path from a side of
said first reflective section remote from said second reflective section
is approximately half of a difference between said first and second
optical path lengths.

5. An apparatus according to claim 4, wherein said optical component
includes a spacer disposed between said first and second reflective
sections, said spacer being transmissive to said second wavelength.

6. An apparatus according to claim 4, wherein said optical component
includes a substrate, said first and second reflective sections being
provided on one side of said substrate, with said second reflective
section between said substrate and said first reflective section.

7. An apparatus according to claim 4, wherein said optical component
includes a substrate that is transmissive to each of said first and
second wavelengths, said first and second reflective sections being
provided on one side of said substrate, with said first reflective
section between said substrate and said second reflective section.

8. An apparatus according to claim 4, wherein said optical component
includes a substrate, said second reflective section including a
reflective surface provided on said substrate.

9. An apparatus according to claim 4, wherein said first reflective
section includes a thin-film optical coating having a plurality of
layers.

10. An apparatus according to claim 9, wherein said second reflective
section includes a thin-film optical coating having a plurality of
layers.

11. A method of optical path length adjustment, comprising: routing first
radiation and second radiation respectively having first and second
wavelengths that are different along respective first and second optical
paths; reflecting said first radiation with an optical component while
said first radiation is traveling along said first optical path; and
reflecting said second radiation with said optical component while said
second radiation is traveling along said second optical path, said
optical component causing a first optical path length traveled by said
first radiation along said first optical path from arrival at to
departure from said optical component to be shorter than a second optical
path length traveled by said second radiation along said second optical
path from arrival at to departure from said optical component.

12. An apparatus according to claim 11, including: routing third
radiation with a third wavelength different from said first and second
wavelengths along a third optical path; and reflecting said third
radiation with said optical component while said third radiation is
traveling along said third optical path, said optical component causing a
third optical path length traveled by said third radiation along said
third optical path from arrival at to departure from said optical
component to be longer than said second optical path length.

13. An apparatus according to claim 11, wherein said reflecting of said
first radiation is carried out using a first reflective section of said
optical component that is reflective to said first wavelength and
transmissive to said second wavelength; wherein said reflecting of said
second radiation is carried out using a second reflective section of said
optical component that is reflective to said second wavelength; and
wherein said routing of said second radiation includes causing said
second radiation to travel along said second optical path through said
first reflective section toward said second reflective section, and to
thereafter travel away from said second reflective section along said
second optical path through said first reflective section.

14. An apparatus according to claim 13, including configuring said
optical component so that an optical distance to said second reflective
section along said second optical path from a side of said first
reflective section remote from said second reflective section is
approximately half of a difference between said first and second optical
path lengths.

15. An apparatus according to claim 14, wherein said configuring includes
providing a spacer between said first and second reflective sections,
said spacer being transmissive to said second wavelength.

16. An apparatus according to claim 14, wherein said configuring
includes: providing a substrate; and locating said first and second
reflective sections on one side of said substrate, with said second
reflective section between said substrate and said first reflective
section.

17. An apparatus according to claim 14, wherein said configuring includes
providing a substrate that is transmissive to each of said first and
second wavelengths; and locating said first and second reflective
sections on one side of said substrate, with said first reflective
section between said substrate and said second reflective section.

18. An apparatus according to claim 14, wherein said configuring includes
providing a substrate having thereon a reflective surface that serves as
said second reflective section.

19. An apparatus according to claim 14, including configuring said first
reflective section to have a thin-film optical coating with a plurality
of layers.

20. An apparatus according to claim 19, including configuring said second
reflective section to have a thin-film optical coating with a plurality
of layers.

Description:

FIELD OF THE INVENTION

[0001] This invention relates in general to optical systems and, more
particularly, to techniques that compensate for dispersion in optical
systems.

BACKGROUND

[0002] In an optical system, a transmissive optical element such as a lens
or window will often exhibit slightly different indexes of refraction to
respective different wavelengths of radiation. This is commonly referred
to as the dispersion characteristic of the component. When different
wavelengths pass through the optical component, they can be displaced
relative to each other. As a result, they will typically have different
optical path lengths within the component, and will ultimately focus at
different locations along an optical axis. If the different wavelengths
pass through multiple optical components, the effects of different
optical path lengths for respective wavelengths can be cumulative.

[0003] To design a viable optical system that functions at two or more
wavelengths, it is generally necessary to minimize dispersive effects.
However, minimizing dispersive effects adds a significant level of
complexity to the design process, particularly as the system's range of
operational wavelengths increases. Not only must suitable optical
materials such as glasses be selected for the various optical components,
but suitable shapes, a suitable order, and suitable positions must also
be determined. The materials, shapes, order and positions of the optical
components interact, and thus a designer usually must spend a great deal
of time balancing all of these interacting factors in order to find an
acceptable combination. Although pre-existing techniques of this general
type have been generally adequate for their intended purposes, they have
not been satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] A better understanding of the present invention will be realized
from the detailed description that follows, taken in conjunction with the
accompanying drawings, in which:

[0005] FIG. 1 is a diagrammatic view of an apparatus that is an optical
system, and that includes an optical component.

[0006]FIG. 2 is a diagrammatic fragmentary sectional side view showing
the optical component of FIG. 1 in an enlarged scale, and providing
additional detail regarding its structure.

[0007] FIG. 3 is a diagrammatic fragmentary sectional side view of an
optical component that is an alternative embodiment of the optical
component of FIGS. 1 and 2.

[0008]FIG. 4 is a diagrammatic fragmentary sectional side view of an
optical component that is an alternative embodiment of the optical
component of FIG. 3.

[0009] FIG. 5 is a diagrammatic fragmentary side view of an optical
component that is a further alternative embodiment of the optical
component of FIG. 3.

[0010]FIG. 6 is a diagrammatic fragmentary side view of an optical
component that is a further alternative embodiment of the optical
component of FIG. 1.

[0011] FIG. 7 is a diagrammatic fragmentary side view of an optical
component that is yet another alternative embodiment of the optical
component of FIG. 1.

[0012] FIG. 8 is a diagrammatic view of an apparatus in the form of an
optical system that is an alternative embodiment of the optical system of
FIG. 1, and that includes an optical component.

[0013]FIG. 9 is a diagrammatic fragmentary sectional side view showing
the optical component of FIG. 8 in an enlarged scale, and providing
additional detail about its structure.

DETAILED DESCRIPTION

[0014] FIG. 1 is a diagrammatic view of an apparatus that is an optical
system 10. The following discussion of the optical system 10 mentions
various thicknesses, and in this regard it is relevant to understand the
difference between physical thickness and optical thickness. More
specifically, optical thickness "t" is defined as refractive index "n"
multiplied by the actual physical thickness "d", or in other words t=nd.
Thus, if two different materials have identical physical thicknesses but
different refractive indices, they will have different optical
thicknesses. Conversely, if two materials have identical optical
thicknesses and different refractive indices, they will have different
physical thicknesses.

[0015] Focusing specifically now on the optical system 10, an element 12
that is a piece of fused silica has a physical thickness of 1 cm. Two
beams 16 and 17 of laser light pass through the element 12, and have
different wavelengths. In particular, the beam 16 has a wavelength of
1064 nm, and the beam 17 has a wavelength of 1542 nm. These two specific
wavelengths are mentioned here only for the sake of example. The beams 16
and 17 could alternatively have other wavelengths, provided the
wavelength of beam 16 is different from the wavelength of beam 17. The
beams 16 and 17 would typically be coincident but, for purposes of
clarity in explaining the optical system 10, they are depicted in FIG. 1
as being slightly spaced.

[0016] The element 12 of fused silica exhibits slightly different indexes
of refraction with respect to the respective wavelengths of each of the
beams 16 and 17. In particular, the element 12 exhibits a refractive
index of 1.44963 with respect to the 1064 nm wavelength of beam 16, and
exhibits a refractive index of 1.44412 with respect to the 1542 nm
wavelength of beam 17. Due to this slight difference in refractive
indexes, the beams 16 and 17 will be displaced with respect to each other
as they travel through the element 12, thereby causing the optical path
length traveled by the beam 16 within element 12 to be about 55.1 μm
longer than the optical path length traveled by beam 17 within element
12. As a result, the beams 16 and 17 will ultimately focus at different
points along an optical axis.

[0017] The fact that different wavelengths are influenced differently by
the element 12 is commonly referred to as the dispersion characteristic
of the element 12. For simplicity, FIG. 1 shows only a single element 12
with a dispersion characteristic that exerts a different influence on
each of the beams 16 and 17. However, the element 12 is just one example
of optics having a dispersion characteristic. In place of the element 12,
some other optical component, or a combination of optical components,
could produce dispersion.

[0018] The optical system 10 includes another optical component 21 that is
provided specifically to counteract and thus compensate for the
dispersive effect of the element 12. In more detail, and as noted above,
the optical path length traveled by beam 16 within element 12 is about
55.1 μm longer than the optical path length traveled by beam 17 within
element 12. The optical component 21 introduces an optical path length
differential that is approximately equal and opposite to that introduced
by the element 12. Thus, the optical path length traveled by beam 17
within optical component 21 is about 55.1 μm longer than the optical
path length traveled by beam 16 within the optical component.

[0019] For clarity, the optical component 21 is not shown to scale in FIG.
1. The optical component 21 includes a substrate 24 that, in the
disclosed embodiment, is made of an optical glass. More particularly, the
substrate 24 in FIG. 1 is made from a material that is commonly known as
BK7 glass, and that can be obtained commercially from a number of
sources, one of which is Red Optronics of Mountain View, Calif.
Alternatively, however, the substrate 24 could be made of any other
suitable material.

[0020] The optical component 21 also includes a reflective section 26 that
is provided on one side surface of the glass substrate 24, and a further
reflective section 27 that is provided on a side of the reflective
section 26 opposite from substrate 24. The reflective section 26 is
reflective to the beam 17, or in other words is reflective to radiation
within a range of wavelengths that includes the wavelength 1542 nm. The
reflective section 27 passes the beam 17, and reflects the beam 16. In
other words, the reflective section 27 is transmissive to radiation
within a range of wavelengths that includes the wavelength 1542 nm, and
is reflective to radiation within a different range of wavelengths that
includes the wavelength 1064 nm. The reflective section 27 has an optical
thickness of approximately 27.55 μm, or in other words half of 55.1
μm.

[0021] It will be noted from FIG. 1 that, although the beam 16 is
reflected by the reflective section 27, the beam 17 passes through the
reflective section 27, is reflected by the reflective section 26, and
then passes back through the reflective section 27. Thus, due to the fact
that the beam 17 passes through the reflective section 27 twice, the
optical path length of the beam 17 within the optical component 21 is
about 55.1 μm longer than that for the beam 16, thereby counteracting
and compensating for the optical path length differential introduced by
the element 12.

[0022]FIG. 2 is a diagrammatic fragmentary sectional side view showing
the optical component 21 of FIG. 1 in an enlarged scale, and providing
additional detail about its structure. For clarity, the various portions
of the optical component 21 are not shown to scale in FIG. 2. In FIG. 2,
the reflective sections 26 and 27 are each implemented as a multi-layer
thin-film optical filter. The reflective section 26 includes alternating
layers of silicon dioxide (SiO2) and tantalum oxide
(Ta2O5) that each have an optical thickness of 1 quarter-wave
(QW) of the wavelength 1542 nm, or in other words an optical thickness
that is 25% of 1542 nm, and thus 385.5 nm. Reflective section 26 includes
20 of the SiO2 layers, and 20 of the Ta2O5 layers.
Similarly, the reflective section 27 includes alternating layers of
SiO2 and Ta2O5 that each have an optical thickness of 1 QW
of the wavelength 1064 nm, or in other words an optical thickness that is
25% of 1064, and thus 266 nm. Reflective section 27 includes 50 of the
SiO2 layers, and 50 of the Ta2O5 layers.

[0023] The optical component 21 further includes two thin layers 31 and 32
that are provided on the outer side of the reflective section 27. Layers
31 and 32 are so thin that they are not shown separately in FIG. 1, but
they are shown diagrammatically in FIG. 2. Layer 31 is a single layer of
Ta2O5 having an optical thickness of 1.3 QW of the wavelength
1542 nm. Layer 32 is a single layer of SiO2 having an optical
thickness of 0.7 QW of the wavelength 1542 nm. Absent the layers 31 and
32, the reflective section 27 would reflect a small portion of the energy
of the beam 17 having the wavelength of 1542 nm. The layers 31 and 32
serve to prevent any significant reflection of the radiation of beam 17,
so that virtually all the radiation of beam 17 enters the optical
component 21 and travels through the reflective section 27 to the
reflective section 26.

[0024] The specific materials and layers shown in FIG. 2 represent only
one possible configuration for the optical component 21. Other
configurations of materials and/or other layers could alternatively be
utilized. For example, if the beams 16 and 17 of FIG. 1 happened to have
wavelengths different from the two exemplary wavelengths of 1064 nm and
1542 nm discussed above, the materials and/or structure of the optical
component 21 would be adjusted so as to provide appropriate optical
path-length compensation for the specific wavelengths involved.

[0025] The particular construction shown in FIG. 2 for the optical
component 21 assumes that the beams 16 and 17 of FIG. 1 each impinge on
the optical component 21 at an angle of approximately 10° with
respect to a not-illustrated reference line extending perpendicular to
the layers in the optical component 21. In some other application where
radiation will impinge on the optical component 21 at a different angle,
the combination of materials and/or layering can be adjusted to optimize
performance for that angle.

[0026] The specific construction shown in FIG. 2 for the optical component
21 reduces the overall optical path length differential between beams 16
and 17 to about 0.3 μm. In order to reduce this differential to
approximately zero, an additional layer can be added between the
reflective sections 26 and 27, and can have an optical thickness of 0.15
μm (0.3 μm divided by 2). In this regard, FIG. 3 is a diagrammatic
fragmentary sectional side view of an optical component 51 that is an
alternative embodiment of the optical component 21 of FIGS. 1 and 2.
Identical or equivalent portions are identified with the same reference
numerals, and the following discussion will focus on the differences. For
clarity, the optical component 51 is not shown to scale in FIG. 3.

[0027] The optical component 51 is effectively identical to the optical
component 21, except that an additional layer 53 has been added between
the reflective sections 26 and 27, and serves as a spacer layer. The
spacer layer 53 is a single layer of SiO2 having an optical
thickness of 0.1 QW of the wavelength 1542 nm. As noted above, if the
optical component 51 is substituted for the optical component 21 in FIG.
1, the optical component 51 will almost completely counteract and
compensate for the optical path length differential introduced between
the beams 16 and 17 by the element 12.

[0028] In another variation, the optical thickness of the spacer layer 53
can be increased, and the number of pairs of alternating layers in the
reflective section 27 can be decreased. As an example of this approach,
FIG. 4 is a diagrammatic fragmentary sectional side view of an optical
component 61 that is an alternative embodiment of the optical component
51 of FIG. 3. Identical or equivalent portions are identified with the
same reference numerals, and the following discussion focuses on the
differences. For clarity, the optical component 61 is not shown to scale
in FIG. 4. In the optical component 61, a reflective section 63 has
replaced the reflective section 27 of the optical component 51 in FIG. 3.
The reflective section 63 differs from the reflective section 27 in that
it has fewer pairs of alternating layers. In particular, the reflective
section 63 has 30 pairs of alternating layers, rather than 50 pairs of
alternating layers. More specifically, the reflective section 63 includes
30 layers of SiO2 that alternate with 30 layers of Ta2O5,
each of these 60 layers having an optical thickness of 1 QW of the
wavelength 1064 nm.

[0029] In the optical component 61 of FIG. 4, two spacer layers 67 and 68
have replaced the spacer layer 53 of the optical component 51 in FIG. 3.
The spacer layer 67 is a single layer of SiO2 having an optical
thickness of 0.1 QW of the wavelength 1542 nm, and the spacer layer 68 is
a single layer of SiO2 having an optical thickness of 40 QW of the
wavelength 1064 nm. Because the spacer layers 67 and 68 are each made of
SiO2 and physically engage each other, as a practical matter they
collectively form only a single layer of SiO2. They are shown as
separate layers in FIG. 4 only to facilitate a clearer understanding of
the optical component 61. A broken line is provided between layers 67 and
68 to diagrammatically indicate that they are actually just different
portions of a single layer.

[0030] FIG. 5 is a diagrammatic fragmentary side view of an optical
component 71 that is a further alternative embodiment of the optical
component 51 of FIG. 3. Identical or equivalent parts are identified with
the same reference numerals, and the following discussion focuses on the
differences. For clarity, the optical component 71 is not shown to scale
in FIG. 5. The optical component 71 of FIG. 5 is intended for use in an
application where a relatively large optical path length differential
exists between the two radiation beams 16 and 17. Because the optical
path length differential is relatively large, and a relatively large
optical path length correction is needed, a relatively large spacer layer
73 is provided between the reflective sections 26 and 27, the spacer
layer 73 having an optical thickness that is appropriate to obtain the
desired optical path length correction. Due to the fact that the spacer
layer 73 is relatively thick, the spacer layer 73 also serves as a
substrate, and so the glass substrate shown at 24 in FIG. 3 is omitted
from the optical component 71 of FIG. 5.

[0031]FIG. 6 is a diagrammatic fragmentary side view of an optical
component 81 that is a further alternative embodiment of the optical
component 21 of FIG. 1. Identical or equivalent portions are identified
with the same reference numerals, and the following discussion focuses on
the differences. For clarity, the optical component 81 is not shown to
scale in FIG. 6. The optical component 81 includes the same glass
substrate 24 and the same reflective sections 26 and 27 as the optical
component 21, but they are arranged differently. In particular, the
reflective sections 26 and 27 are provided on opposite sides of the
substrate 24, with the reflective section 27 disposed between the
reflective section 26 and the substrate 24. Between the reflective
section 27 and the substrate 24 there are two layers that are
functionally equivalent to the layers 31 and 32 in FIG. 3, but have
optical thicknesses different from those of the layers 31 and 32, and are
so thin that they are not visible in FIG. 6. The beam 16 travels through
the substrate 24 to the reflective section 27, where it is reflected and
then travels back through the substrate 24. The beam 17 travels through
the substrate 24 and the reflective section 27 to the reflective section
26, where it is reflected and travels back through the reflective section
27 and the substrate 24. In the embodiment of FIG. 6, if the material of
the substrate 24 happens to introduce any dispersive effect, the
reflective sections 26 and 27 can be adjusted to compensate for that.

[0032] FIG. 7 is a diagrammatic fragmentary side view of an optical
component 91 that is yet another alternative embodiment of the optical
component 21 of FIG. 1. Identical or equivalent portions are identified
with the same reference numerals, and the following discussion focuses on
the differences. For clarity, the optical component 91 is not shown to
scale in FIG. 3. The optical component 91 includes the reflective section
27 of the optical component 21, but the reflective section 26 is omitted,
and the glass substrate 24 is replaced with a substrate 93 that is made
of aluminum and that has a polished external surface 94 which is
reflective to the 1542 nm wavelength of the beam 17. The reflective
section 27 is provided on the surface 94. The substrate 93 could
alternatively be made from some other type of metal, or from some other
suitable material that can have a surface which is highly reflective to
the relevant wavelength(s). As another alternative, the substrate 93
could be made from glass or some other non-reflective material, and could
have on the surface 94 a thin layer of a metal or some other material
that is highly reflective to the relevant wavelength(s).

[0033] For simplicity, the foregoing discussion of FIGS. 1-7 has been
presented in the context of two specific wavelengths that are 1064 nm and
1542 nm. However, the beams 16 and 17 could alternatively involve
mutually exclusive ranges of wavelengths that have 1064 nm and 1524 nm as
respective center wavelengths. Moreover, the invention is not limited to
two specific wavelengths, or even two ranges of wavelengths, but could
instead involve three or more wavelengths, and/or three or more
wavelength ranges. As one example, FIG. 8 is a diagrammatic view of an
apparatus in the form of an optical system 110 that is an alternative
embodiment of the optical system 10 of FIG. 1. Identical or equivalent
portions are identified with the same reference numerals, and the
following discussion focuses on the differences.

[0034] In the system 110 of FIG. 8, an optical element 112 has been
substituted for the fused silica element 12 of the system 10 in FIG. 1.
The optical element 112 is made of BK7 glass, and has a physical
thickness of 1 cm. Three radiation beams 116, 117 and 118 of laser light
propagate through the element 112. The beams 116-118 would normally be
coincident, but for clarity they are shown in FIG. 8 as being slightly
spaced. The beam 116 has a wavelength of 443 nm, the beam 117 has a
wavelength of 539 nm, and the beam 118 has a wavelength of 653 nm. These
three wavelengths correspond to the basic colors of red, green and blue.
Alternatively, the beams 116-118 could each involve a range of
wavelengths, where the respective indicated wavelength is the center
wavelength of the range. The optical path length of the beam 116 within
element 112 will be approximately 69.1 μm longer than the optical path
length of beam 117 within the element, and the optical path length of
beam 117 within the element will be approximately 46.5 μm longer than
the optical path length of the beam 118 within the element.

[0035] An optical component 121 influences the relative optical path
lengths of the beams 116, 117 and 118 in a manner that is equal and
opposite to the optical path length differential introduced by the
element 112. For clarity, the optical component 121 is not shown to scale
in FIG. 8. The optical component 121 includes the glass substrate 24, and
three reflective sections 126, 127 and 128 provided on a side of the
substrate 24 optically nearest the element 112. The reflective section
128 reflects the beam 116, and passes the beams 117 and 118. The
reflective section 128 has an optical thickness of 34.55 μm, which is
half the optical path-length differential of 69.1 μm between the beams
116 and 117. The reflective section 127 reflects the beam 117, and passes
the beam 118. The reflective section 127 has an optical thickness of
23.25 μm, which is half the 46.5 micron optical path length
differential between the beams 117 and 118. The reflective section 126
reflects the beam 118.

[0036]FIG. 9 is a diagrammatic fragmentary sectional side view showing
the optical component 121 of FIG. 8 in an enlarged scale, and providing
additional detail about its structure. For clarity, the optical component
121 is not shown to scale in FIG. 9. In FIG. 9, the reflective section
126 has 20 layers of SiO2 that alternate with 20 layers of
Ta2O5, where each of these 40 layers has an optical thickness
of 1 QW of the wavelength of 653 nm. The reflective section 127 has 43
layers of SiO2 that each have an optical thickness of 1.4 QW of
wavelength 539 nm, and that alternate with 43 layers of Ta2O5
that each have an optical thickness of 0.6 QW of wavelength 539 nm. The
reflective section 128 has 79 layers of SiO2 that each have an
optical thickness of 1 QW of wavelength 443 nm, and that alternate with
79 layers of Ta2O5 that each have an optical thickness of 1 QW
of wavelength 443 nm. The reflective section 126 reflects a waveband of
approximately 610 nm to 700 nm. The reflective section 127 reflects a
waveband of approximately 500 nm to 600 nm while passing the waveband of
approximately 610 nm to 700 nm. The reflective section 128 reflects a
waveband of approximately 400 nm to 490 nm, while passing a waveband of
approximately 500 nm to 700 nm.

[0037] One practical application for the optical component 121 of FIGS. 8
and 9 is a not-illustrated color projector of the type that has multiple
interchangeable lens assemblies. Each lens assembly has a respective
different focal length, and has different optical characteristics. One
such optical characteristic is dispersion, which affects the location at
which each wavelength focuses along an optical axis. Each of the multiple
different lens assemblies could include a respective optical component of
the general type shown at 121 in FIGS. 8 and 9, where the multiple
reflective sections of each such component would be configured to effect
the appropriate optical path length adjustments needed for that
particular lens assembly.

[0038] In each of the optical components 21, 51, 61, 71, 81, 91 and 121
respectively shown in FIGS. 2, 3, 4, 5, 6, 7 and 9, more than 99% of the
energy of each wavelength will be reflected if the layers in the
reflective sections are all of good quality. The theoretical performances
are more than 99.9%.

[0039] In the disclosed embodiments, the minimum optical path length
differential that can reasonably be corrected is limited by the need to
have a high degree of reflection for one wavelength range in each
reflective section that must also pass one or more other wavelength
ranges. As a reflective section gets thinner, its reflectivity decreases.
For visible light, a reasonable minimum optical thickness is
approximately 4 μm, depending on what is considered to be an
acceptable loss for the particular application.

[0040] Although selected embodiments have been illustrated and described
in detail, it should be understood that a variety of substitutions and
alterations are possible without departing from the spirit and scope of
the present invention, as defined by the claims that follow.